/*
 * Copyright (c) 1996, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package java.io;


/**
 * Writes text to a character-output stream, buffering characters so as to
 * provide for the efficient writing of single characters, arrays, and strings.
 *
 * <p> The buffer size may be specified, or the default size may be accepted.
 * The default is large enough for most purposes.
 *
 * <p> A newLine() method is provided, which uses the platform's own notion of
 * line separator as defined by the system property <tt>line.separator</tt>.
 * Not all platforms use the newline character ('\n') to terminate lines.
 * Calling this method to terminate each output line is therefore preferred to
 * writing a newline character directly.
 *
 * <p> In general, a Writer sends its output immediately to the underlying
 * character or byte stream.  Unless prompt output is required, it is advisable
 * to wrap a BufferedWriter around any Writer whose write() operations may be
 * costly, such as FileWriters and OutputStreamWriters.  For example,
 *
 * <pre>
 * PrintWriter out
 *   = new PrintWriter(new BufferedWriter(new FileWriter("foo.out")));
 * </pre>
 *
 * will buffer the PrintWriter's output to the file.  Without buffering, each
 * invocation of a print() method would cause characters to be converted into
 * bytes that would then be written immediately to the file, which can be very
 * inefficient.
 *
 * <pre>
 *  BufferedWriter的思想很简单：
 *      就是将数据写入到 缓冲区中（ch字符数组中），当缓冲区满的时候或者用户调用flush()方法的时候，
 *   它就会把 缓冲区中的数据 写入到 输出流中。具体的实现逻辑看 {@link #write(String)}
 * </pre>
 *
 * @see PrintWriter
 * @see FileWriter
 * @see OutputStreamWriter
 * @see java.nio.file.Files#newBufferedWriter
 *
 * @author      Mark Reinhold
 * @since       JDK1.1
 */

public class BufferedWriter extends Writer {

    // 下层输出流
    private Writer out;

    /**
     * <pre>
     * 缓冲区，更确切的来说应该叫【专门用来保存 "缓冲输出流"数据的字符数组】。
     * 这个字符数组就是 BufferedWriter的核心。
     * </pre>
     */
    private char cb[];
    /**
     * nChars：cb缓冲区中字符的总数。<br />
     * nextChar：是下一个要写入的字符在cb缓冲区中的位置。<br />
     *      nextChar在构造器中初始化为0; 当缓冲区满时，nextChar又会被重置为0。
     */
    private int nChars, nextChar;
    // 缓冲区默认大小，8192byte = (8192 ÷ 1024)K = 8K
    private static int defaultCharBufferSize = 8192;

    /**
     * Line separator string.  行分隔符
     * This is the value of the line.separator
     * property at the moment that the stream was created.
     */
    private String lineSeparator;

    /**
     * Creates a buffered character-output stream that uses a default-sized
     * output buffer.
     *
     * @param  out  A Writer
     */
    public BufferedWriter(Writer out) {
        this(out, defaultCharBufferSize);
    }

    /**
     * Creates a new buffered character-output stream that uses an output
     * buffer of the given size.
     *
     * @param  out  A Writer
     * @param  sz   Output-buffer size, a positive integer
     *              输出缓冲区大小
     *
     * @exception  IllegalArgumentException  If {@code sz <= 0}
     */
    public BufferedWriter(Writer out, int sz) {
        super(out);
        if (sz <= 0)
            throw new IllegalArgumentException("Buffer size <= 0");
        this.out = out;
        // 缓冲区
        cb = new char[sz];
        nChars = sz;
        nextChar = 0;

        lineSeparator = java.security.AccessController.doPrivileged(
            new sun.security.action.GetPropertyAction("line.separator"));
    }

    /** Checks to make sure that the stream has not been closed */
    private void ensureOpen() throws IOException {
        if (out == null)
            throw new IOException("Stream closed");
    }

    /**
     * <pre>
     * Flushes the output buffer to the underlying character stream, without flushing the stream itself.
     * 将 输出缓冲区 中的数据 刷新到 Writer（{@link #out}）中。
     * This method is non-private only so that it may be invoked by PrintStream.
     * 1. 作用
     *    强制清空缓冲区：将缓冲区中的数据 立即写入目标（比如，文件、网络流等）。
     *    确保数据持久化：调用后，数据会达到目标设备。
     * 2. 特点
     *    同步操作：可能引发实际的I/O操作，性能开销较高。
     *    必要性：在需要确保数据不丢失的情况（比如，关键日志、实时通信）时显式调用。
     * 3. 该方法在什么时候会被调用？
     *    ① 缓冲区的自动刷新
     *       write()方法：缓冲区满时，就会自动调用该方法。
     *    ② 关闭流时的隐式刷新
     *       close()方法：在关闭流时，也会自动调用该方法，确保剩余的数据能够写入目标。
     *    ③ 需要显示调用flush()的场景
     *       实时性要求高：比如，日志文件需要及时查看最新记录。
     *       网络通信：确保数据立即发送到对端。
     * </pre>
     */
    void flushBuffer() throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (nextChar == 0)
                return;
            // 使用下层输出流（out）的write()方法，将 ch缓冲区 中的数据写入到 输出流中
            out.write(cb, 0, nextChar);
            nextChar = 0;
        }
    }

    /**
     * Writes a single character. 写入单个字符，在缓冲区满的时候需要将缓冲区中的字符写入到下层输出流中
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void write(int c) throws IOException {
        synchronized (lock) {
            ensureOpen();
            // 如果 缓冲区满了，则清空缓冲区，将缓冲区的数据写入到 输出流中。
            if (nextChar >= nChars)
                flushBuffer();
            cb[nextChar++] = (char) c;
        }
    }

    /**
     * Our own little min method, to avoid loading java.lang.Math if we've run
     * out of file descriptors and we're trying to print a stack trace.
     */
    private int min(int a, int b) {
        if (a < b) return a;
        return b;
    }

    /**
     * Writes a portion of an array of characters.
     *
     * <p> Ordinarily this method stores characters from the given array into
     * this stream's buffer, flushing the buffer to the underlying stream as
     * needed.  If the requested length is at least as large as the buffer,
     * however, then this method will flush the buffer and write the characters
     * directly to the underlying stream.  Thus redundant
     * <code>BufferedWriter</code>s will not copy data unnecessarily.
     *
     * @param  cbuf  A character array
     * @param  off   Offset from which to start reading characters
     * @param  len   Number of characters to write
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void write(char cbuf[], int off, int len) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if ((off < 0) || (off > cbuf.length) || (len < 0) ||
                ((off + len) > cbuf.length) || ((off + len) < 0)) {
                throw new IndexOutOfBoundsException();
            } else if (len == 0) {
                return;
            }

            if (len >= nChars) {
                /* If the request length exceeds the size of the output buffer,
                   flush the buffer and then write the data directly.  In this
                   way buffered streams will cascade harmlessly. */
                flushBuffer();
                out.write(cbuf, off, len);
                return;
            }

            int b = off, t = off + len;
            while (b < t) {
                int d = min(nChars - nextChar, t - b);
                System.arraycopy(cbuf, b, cb, nextChar, d);
                b += d;
                nextChar += d;
                if (nextChar >= nChars)
                    flushBuffer();
            }
        }
    }

    /**
     * <pre>
     *  Writes a portion of a String. 写入字符串s的一部分。
     *  从字符串s的off位置开始，写入长度是len个字符。
     * 1. 该方法的作用
     *    将数据（字符、字符数组 或 字符串）写入缓冲区，而不是直接写入磁盘或者目标设备。
     * 2. 该方法的特点
     *    高效性：减少频繁的 I/O操作（比如，磁盘写入），提升性能。
     *    无即时持久化：数据可能暂时停留在缓冲区，没有真正的写入目标（比如，文件）。
     * </pre>
     *
     * <p> If the value of the <tt>len</tt> parameter is negative then no
     * characters are written.  This is contrary to the specification of this
     * method in the {@linkplain java.io.Writer#write(java.lang.String,int,int)
     * superclass}, which requires that an {@link IndexOutOfBoundsException} be
     * thrown.
     *
     * @param  s     String to be written
     * @param  off   Offset from which to start reading characters
     * @param  len   Number of characters to be written
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void write(String s, int off, int len) throws IOException {
        synchronized (lock) {
            ensureOpen();

            int b = off, t = off + len;
            while (b < t) {
                int d = min(nChars - nextChar, t - b);
                // 将 字符串s中从b开始一直到b+d的字符 写入到 缓冲区cb中（从缓冲区cb的nextChar位置开始写）。
                s.getChars(b, b + d, cb, nextChar);
                b += d;
                nextChar += d;
                // 当 nextChar >= nChars 时，说明 ch缓冲区 已满，这个时候就需要 将缓冲区中的数据写入到 输出流中。
                if (nextChar >= nChars)
                    flushBuffer();
            }
        }
    }

    /**
     * Writes a line separator.  The line separator string is defined by the
     * system property <tt>line.separator</tt>, and is not necessarily a single
     * newline ('\n') character.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void newLine() throws IOException {
        write(lineSeparator);
    }

    /**
     * Flushes the stream.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void flush() throws IOException {
        synchronized (lock) {
            flushBuffer();
            out.flush();
        }
    }

    @SuppressWarnings("try")
    public void close() throws IOException {
        synchronized (lock) {
            if (out == null) {
                return;
            }
            try (Writer w = out) {
                flushBuffer();
            } finally {
                out = null;
                cb = null;
            }
        }
    }
}
